The invention concerns a device with an electrode for the formation of a ball at the end of a wire.
Such devices are used on so-called Wire Bonders. A Wire Bonder is a machine with which semiconductor chips are wired after mounting on a substrate. The Wire Bonder has a capillary which is clamped to the tip of a horn. The capillary serves to secure the wire to a connection point on the semiconductor chip and to a connection point on the substrate as well as to guide the wire between the two connection points. On producing the wire connection between the connection point on the semiconductor chip and the connection point on the substrate, the end of the wire protruding out of the capillary is first melted into a ball. Afterwards, the wire ball is secured to the connection point on the semiconductor chip by means of pressure and ultrasonics. In doing so, ultrasonics is applied to the horn from an ultrasonic transducer. This process is called ball bonding. The wire is then pulled through to the required length, formed into a wire loop and welded to the connection point on the substrate. This last part of the process is called wedge bonding. After securing the wire to the connection point on the substrate, the wire is torn off and the next bonding cycle can begin.
In order to form the end of the wire protruding out of the capillary into a ball, a high DC voltage is applied between the wire and the electrode so that an electrical spark occurs which melts the wire. The voltage at which the electrical breakdown of the ionised air takes place between the wire and the electrode and the spark is created is designated as sparking voltage.
Today, three types of devices are known for the formation of a wire ball at the end of a wire which prevail on the market. These three types are explained based on FIGS. 1 to 3. With the first type (FIG. 1), a flat electrode 1 is swivelled from the side under the capillary 3 which guides the wire 2. With this arrangement of electrode and wire, the electrical spark 4 is formed in the longitudinal direction of the wire. The electrical spark 4 therefore runs symmetrically to the wire. The advantage of this type is that the symmetry of the formed ball is comparatively high. On the other hand, the disadvantage is that the swivelling in and out of the electrode costs time. Furthermore, the swivelling in and out of the electrode can stimulate oscillations of the bondhead.
With the second type (FIG. 2), an electrode 1 with a tip is arranged laterally offset underneath the capillary. With this arrangement, the electrical spark 4 runs non-symmetrically to the longitudinal axis of the wire which tends to lead to the formation of asymmetrical wire balls. However, the asymmetry of these wire balls has a predominant direction. This makes it possible to reduce the asymmetry with additional measures. In addition, this arrangement requires a higher sparking voltage in comparison to the first type. With the same vertical distance to the downholder plate which is located immediately below the device and holds the connection fingers on the substrate in position, this leads to a greater occurrence of electrical discharges on the downholder plate.
With the third type (FIG. 3), the electrode 1 is a rotationally symmetrical ring electrode. After formation of the wire ball, the capillary 3 is lowered down through the electrode. The advantage of this arrangement is that the voltage necessary for creating the electrical spark is lower than with the first two types. The disadvantage is that the place where the electrical spark 4 is created constantly changes. Wire balls formed with this arrangement also have a tendency towards asymmetries, however these asymmetries have no predominant direction.
A further device for the formation of a wire ball is known from the U.S. Pat. No. 5,263,631. This device has three pointed electrodes which are separately electrically controlled in order to regulate the currents flowing through the individual electrodes.
The object of the invention is to develop a device for the formation of a ball at the end of a wire which is distinguished by a low sparking voltage and with which wire balls can be formed which are symmetrical to the longitudinal axis of the wire and the size of which varies as little as possible.
For a device for the formation of a ball at the end of a wire, the invention consists in using an electrode with a longitudinal shape which can be swivelled in and out and which is attached directly, or indirectly via a connection part, to the shaft of a motor so that, when swivelling in and out, the electrode is essentially turned on its longitudinal axis. (However, the rotational axis and the longitudinal axis of the electrode do not necessarily coincide.) The electrode is preferably divided into three sections at angles to each other. The shape of the electrode can be broadly designated as being s-shaped. The mass inertia of the electrode is low as regards the rotational axis. The electrode can therefore be rotated with little expenditure of energy. On swivelling in and out, the electrode is turned back and forth between a lower limit position in which a surface necessary for spark formation, the so-called firing lug (which in the art is also known as flame off plate), is located underneath the capillary of the Wire Bonder, and an upper limit position in which the firing lug is located laterally elevated next to the capillary.